Techniques for converting an AC voltage to an equivalent DC voltage representing the amplitude of the AC voltage are well-known in many electrical or electronic systems. In the field of ion processing and mass spectrometry, a Radio Frequency (RF) detector uses such techniques to transform the RF voltage to a corresponding signal indicative of the amplitude of the RF voltage as a DC voltage, as part of a closed-loop control circuit to maintain the amplitude of the RF voltage as constant. It will be understood that such a DC voltage is not strictly a DC voltage of a constant amplitude over time. Rather, the DC voltage is a signal having a DC level that indicates the amplitude of the RF voltage, which can be constant over the time but also change gradually over the time depending on the transformed RF voltage as an input.
The closed-loop control circuit may form part of an ion optical device, such as a quadrupole, an ion trap, collision cell or an ion optical lens. Variation in the amplitude of the RF potential may affect the electric field to which the ions are subjected in the ion optical device, causing undesirable effects such as inaccurate measurements and loss of ions.
The current-voltage (I-V) characteristics of the components used in AC to DC conversion will have a significant effect on the DC output level. In particular, active components often used for rectification have current-voltage characteristics that vary non-linearly with respect to temperature. For example, the currently most used semiconductor material is silicon doped with different other chemical elements and combined to form a PN-junction. This junction has a temperature dependency in that the relationship between the voltage across the junction and the current through depends on the junction temperature.
Referring to FIG. 1, there is shown a known relationship between the current through a diode (ID) and the voltage across the diode (UD) and how this varies with respect to temperature. This relationship is shown in both graphical and mathematical forms. Every semiconductor has an intrinsic ohmic part (generally around 15-30Ω for a diode) in its current path, so some power loss will occur, even in normal operation. A consequence of this power loss is that the semiconductor PN-junction or metal-semiconductor Schottky junction of the diode will increase in temperature. Thus, as shown in FIG. 1, such an effect is self-amplifying and a stable condition will not be reached on its own. A diode generally also has a parasitic capacitance of no more than about 1 pF.
Referring next to FIG. 2, there is depicted a typical existing circuit of a RF detector for converting an input AC voltage signal 10 to a corresponding DC voltage 60 using a diode 20, together with a graph illustrating the relationship between the input voltage (Urf) and the power dissipated in the diode (PD). Such a design is used, for example, in the Inductively Coupled Plasma (ICP) mass spectrometer marketed by Thermo Fisher Scientific (TM) under the brand name “iCAP”, models “Q” and “RQ”. The RF detector is normally used in an electronic closed-loop circuit, to hold the amplitude of the RF signal at a desired, constant level. The RF detector comprises: the diode 20; a resistor 30; a capacitor 40; and an inductor 50. The capacitor 40 and inductor 50 form a low pass filter, so that the DC output 60 has a low ripple. Due to the use of a semiconductor (the diode 20) as the rectifying element, this configuration is temperature dependent. The relationship between voltage and the power dissipated in the diode (which is closely linked to the diode temperature, TD) is highly nonlinear. This results in the relationship between the amplitude of the AC input 10 and the DC voltage level 60 being nonlinear with respect to temperature as well.
Hence, temperature effects will cause a DC offset voltage error at the output 60 of the RF detector circuit. This DC offset voltage also has a nonlinear relationship with respect to the incoming AC voltage 10, because a higher incoming voltage 10 will cause a higher current in the diode (and potentially other components), more power loss and therefore more heat in the diode, changing its current-voltage characteristic. This is particularly a problem when high power inputs are provided, such as for an RF potential to be provided to an ion optical device.
It is also known to maintain the circuit at a constant temperature by external temperature control, as considered in U.S. Pat. Nos. 2,221,703 and 2,930,904, for instance. For example, a temperature sensor may be used to detect the temperature and a heater may then increase the temperature if it is below a desired level, by closed-loop feedback. The temperature is typically maintained at a temperature higher than room temperature thereby. The idea behind this approach is to eliminate heat-related error sources in the rectifying component itself. By holding the temperature constant, the voltage from the detector may be kept precise and stable over time, even with temperature changes.
With reference to FIG. 10, there is schematically shown an arrangement of components for an existing ambient temperature-compensated RF detector circuit 500. The circuit 500 comprises: rectifying diodes 520; load resistors 530; a capacitor 540; an inductor 550; heating resistors 560; a transistor 570; an operational amplifier 580; and passive control components 590. These are mounted on a printed circuit board (PCB). In view of the electrical requirements, the components are typically laid out in a symmetrical fashion, indicated by symmetry line 501.
A temperature sensor 595 measures the temperature at a chosen point on the PCB and an according amount of heat is provided to keep the temperature stable. The rectifying diodes 520, load resistors 530, capacitor 540 and inductor 550 are the heated components 510 forming the RF detector (for instance as shown in FIG. 2). The transistor 570, located below the heated components 510 on the PCB, controls the current through the heating resistors 560, which maintain the heat level. The transistor 570 is controlled by the operational amplifier 580 and its associated passive control components 590, to set the heating level. This closed-loop control circuit is located below the actuating transistor 570.
The heating resistors 560 convert the electrical power determined by the current supplied to heat energy to heat the components 510, which they surround. The transport of the heat is mainly realised by thermal conduction through the PCB material, which is typically made from an FR4 material. This arrangement is intended to provide a constant temperature when the ambient temperature is in the range of 15° C. to 35° C.
This approach has several drawbacks. Firstly, the flow of heat through the circuit is non-uniform, making steady-state control difficult. Furthermore, each resistor has its own tolerance, making the amount of heat generated difficult to set. The actuator transistor 570 also represents an extra source of heat, getting hot as a consequence of its parasitic resistance, the high current running through it and the voltage between its collector and emitter. These effects create an uneven temperature on the surface of the PCB, and give a longer time constant to reach thermal equilibrium. In view of Ohm's law and the formula for electrical power (P=I2R), a small change in current can cause a large change in the power dissipated and consequent heating effect. This non-linear relationship makes control even trickier.
The intention of this technique is generally to reduce any influences that room temperature changes might have on the output. In some scenarios, controlling the circuit temperature in this way is less effective, particularly when high power input signals are provided. The heat generated by the circuit (self-heating), especially the diode, may therefore have a significant impact on the performance. The relationship between the current-voltage relationship and temperature may be linear, but is typically non-linear (quadratic or logarithmic). The close proximity of the components makes the relationship even more unpredictable. Moreover, the area of the whole circuit is much larger than the small die size of a component, such as the diode. Hence, the time to make a temperature change in a component can be magnitudes smaller than the time for the entire circuit board temperature to change by the same amount. This makes the use of such ambient temperature compensation techniques to mitigate the self-heating effect very challenging.
Providing a circuit, for instance using a rectifying component (such as a diode) and particularly for operation with high input voltages and/or powers, with a current-voltage characteristic having a reduced (preferably minimal or negligible) temperature dependence and/or a stable operation temperature would therefore be highly advantageous.